There's something about being completely isolated from the world, with nothing but the sound and sensation of running water, that evokes a certain sense of clarity. These profound moments of intuitive thought can bring us epiphanies and realizations that would have otherwise remained shrouded in the mists of our minds.
Well. Then there's moments like these where, instead of solving the problems that plague your life, your brain starts dwelling on the fact that your tie is an arrow pointing at your dick and other deliciously inane things that have otherwise no bearing on your life.
These shower thoughts fall more in the second category.